1. Field of the Invention
The present invention relates to an optical waveguide incorporating submedia.
2. Description of the Background Art
An optical fiber, which belongs to the category of the optical waveguide, is used, for example, as the optical transmission line in an optical communications system. The system usually uses an optical fiber in which doped materials are dispersed with a cylindrically symmetrical concentration distribution in the matrix made of silica glass. The optical fiber has a refractive-index profile determined by the concentration distribution of the doped material. The chromatic dispersion, one of the properties of an optical fiber, is determined by both the material dispersion, which is determined by the composition (matrix and doped material), and the waveguide dispersion, which is determined by the refractive-index profile.
The chromatic dispersion of an optical fiber is designed by properly determining the refractive-index profile. For example, a standard single-mode optical fiber has a chromatic dispersion of 0 ps/nm/km at a wavelength of about 1.3 μm and a chromatic dispersion of 16 to 20 ps/nm/km or so at a wavelength of 1.55 μm. It is also possible to obtain a dispersion-compensating optical fiber having a negative chromatic dispersion at a wavelength of 1.55 μm with an absolute value of several tens in ps/nm/km. Furthermore, it is possible to obtain a dispersion-shifted optical fiber having a chromatic dispersion of a single-digit number in ps/nm/km.
However, when the light travelling in these optical fibers has a large power, the waveform of the light deteriorates due to an optical nonlinear phenomenon. In addition, the cumulative dispersion of the optical fiber also deteriorates the waveform of the light. The reduction in transmission loss in these optical fibers has a limitation due to the presence of the doped material.
Recently, the published Japanese patent application Tokukai 2000-35521 has disclosed an optical fiber whose main medium includes submedia longitudinally extending along the axis of the fiber. According to the cross section perpendicular to the fiber axis, the optical fiber has a core region formed by a hollow and a cladding region composed of a main medium and submedia placed at constant intervals in the main medium. The cladding region forms a photonic bandgap structure. The optical fiber utilizes the Bragg reflection of the light in the photonic bandgap structure to contain the light in the core region surrounded by the cladding region so that the light can be transmitted. The optical fiber having a photonic bandgap structure can form the core region with a hollow. Consequently, the optical fiber holds promise of reducing the transmission loss, nonlinearity, and chromatic dispersion.
However, the above-described optical fiber has a hollow core region extending continuously along its axis. Therefore, foreign matters intruding into the hollow core from the end may deteriorate the optical properties. Accordingly, the optical fiber is required to seal its both ends to prevent foreign matters from entering. This termination work takes time and consumes man power. In addition, the optical fiber having a photonic bandgap structure allows part of the energy of the travelling light to enter the region of the photonic bandgap structure. As a result, the optical fiber cannot sufficiently reduce the transmission loss, nonlinearity, and chromatic dispersion.